1. Field of The Invention
The field of the invention relates to methods for making impermeable belts with smooth inner surfaces and more particularly a belt used in an extended nip press for dewatering a fibrous web formed in a papermaking machine.
2. Description of the Prior Art
Extended nip presses are currently used in the papermaking industry in the formation of paper and kraft products. Many presses of this type use an endless, impervious, oil, abrasion and crush resistant belt which runs against the shoe in the press nip. Early designs used long belts in the range of 25 to 50 feet.
More recent extended nip press designs have been scaled down in size and require belts of much shorter lengths. These recent press designs generally incorporate a circular belt configuration, in the shape of a drum, circumscribing the nip, rather than an expanded rectangular belt configuration as was used in the early designs.
The long belts used on earlier designed machines are made endless and are impregnated with a tough thermoset resin.
The coated surface is then ground to a uniform thickness and polished smooth. The belt is then inverted so the smooth impregnated surface is on the inside. Since during manufacture the outside of the belt is polished, and since it is the inside of the belt which must be smooth for communicating with an oil lubricated press shoe, inversion of the belt is necessary. With the development of the new extended nip presses, the length of belt has been reduced to 13 to 15 feet. Because of this reduced length the previous means of manufacture is inappropriate because a belt of such short length cannot be readily inverted. Not only is the process of inverting a short relatively brittle belt very difficult (if not impossible) but the stresses upon the belt in inverting it could create weak points in the belt leading to failure during use. The extended nip press requires that the belt be uniform in thickness and with the inner surface polished smooth so that it can ride on the oil lubricated pressure shoe which acts as a hydraulic bearing.
Belts used in extended nip presses generally require some sort of fiber reinforcement to withstand the tremendous pressures of up to 6,000 pounds per linear inch in operation. See e.g. U.S. Pat. No. 4,229,253. A belt commonly includes a base fabric or carcass coated with a resinous material. Uniformity of belt thickness as well as uniformity of level of carcass depth within the coating are essential in belts of this type. Processes for coating the carcass usually include the use of two rolls, one tension roll and one drive roll, and a coating or impregnation device. A method for forming a reinforced plastic belt loop is disclosed in U.S. Pat. No. 4,267,139 which involves placing an endless shrinkable carcass in a mold, then filling the mold with a casting plastic. The woven structure is then encapsulated in the cured plastic. A drawback of a belt made by this process is that the carcass, in order to achieve uniform depth within the coating, must be set to contact the inner support core of the mold. Consequently, in the final product, the carcass is disposed adjacent to the internal surface of the belt.